A thin liquid crystal display apparatus which consumes less electric power has adopted an active matrix driving method which uses a thin film transistor (TFT) as an active element of liquid crystal. The liquid crystal display apparatus adopting the active matrix driving method has properties such that a responding speed of the TFT is high, high contrast is produced, and an image with high quality and with high resolution can be obtained. Therefore, the liquid crystal display apparatus is mainly used for a display section of a personal computer, a portable television, etc., and thus its market has been greatly expanded.
As to the TFT, there exist a p-Si TFT using a polysilicon (p-Si) for a semiconductor in a channel section and an a-Si TFT using an amorphous silicon (a-Si) for the semiconductor in the channel section. Since the p-Si TFT consumes less electric power and responds at higher speed compared with the a-Si TFT, it has a good prospect as the active element of the liquid crystal display apparatus which complies with the future multimedia.
In addition, since the p-Si TFT has great mobility of a carrier in silicon when a voltage is applied so as to have a desired electric field, it is possible to produce a driver circuit for driving liquid crystal having a structure that a gate driver and a source driver are arranged together with the TFT element in a picture element of the liquid crystal, namely, so-called "driver monolithic structure".
As shown in FIGS. 21 and 22, the driver circuit for driving liquid crystal with the driver monolithic structure is arranged such that a channel layer 102, a first insulating layer 103, a gate wiring 104, a gate electrode 105, an Si layer 106, a second insulating layer 107 and a source/drain electrode wiring 108 are provided on a glass substrate 101.
Furthermore, the driver circuit for driving liquid crystal with the above arrangement is produced in the following manner.
First, an Si film is formed on the glass substrate 101 by the CVD method, and the Si film is patterned so that the channel layer 102 is formed. Then, the Si film is changed to an amorphous Si film by a solid phase growth method or by irradiation of an excimer laser, and then SiO.sub.2 is formed on the glass substrate 101 by the sputtering method so that the first insulating layer 103 is obtained.
Next, a metallic film such as Al, Ti is formed on the first insulating layer 103, and the metallic film is patterned so that the gate wiring 104 and the gate electrode 105 are formed. The surface of the substrate is doped with ion impurities such as phosphorus, boron with high concentration by using the gate electrode 105 as a mask so that the Si layer 106 of n+ or p+ is formed on the channel layer 102.
Successively, after SiNx as the second insulating layer 107 is formed on the whole surface of the substrate by the sputtering method, a part of the first insulating layer 103 and a part of the second insulating layer 107 are etched so that contact holes 109 . . . are formed, and a part of the gate wiring 104 and a part of the Si layer 106 are exposed.
In addition, metal such as Al, Ti is formed on the substrate where the part of the gate wiring 104 and the part of the Si layer 106 have been exposed and is patterned so that the source/drain electrode wiring 108 is formed. At this time, the source/drain electrode wiring 108 is formed such that it is connected to the gate wiring 104 and the Si layer 106 which have been exposed through the contact hole 109. In such a manner, the driver circuit for driving liquid crystal is formed.
The driver circuit for driving liquid crystal formed in the above manner has the structure that the gate driver and the source driver are formed together with the TFT in the picture element of the liquid crystal, namely the driver monolithic structure. As a result, decrease in a number of IC chips for the drivers and in installing steps can be attempted, thereby making it possible to lower cost of the liquid crystal display apparatus.
As materials used for the gate wiring 104 and the gate electrode 105 of the driver circuit for driving liquid crystal, Ta, Cr, Al or n+Si, etc. having comparatively low resistance is used. In order to solve a problem, such as delay of a signal due to enlargement of a screen, Al which has particularly low resistance is being used as the materials of the gate wiring 104 and the gate electrode 105 recently.
However, in general, when Al is heated, unevenness which is called as hillock is generated. Since the hillock causes dielectric breakdown of the insulating film and thus leakage of an electric current occurs, the generation of the hillock should be suppressed. For this reason, it is considered that Al alloy obtained by adding several % of another material to Al is used. However, the percentage of the material to be added to Al is increased in order to suppress the generation of the hillock on the Al surface, electric resistance becomes high, thereby arising a problem such as the delay of a signal.
In addition, since Al has low chemical resistance, in the case where the gate wiring 104 and the gate electrode 105 are made of Al, the gate wiring 104 and the gate electrode 105 is necessary to be protected.
Therefore, Japanese Unexamined Patent Publication No. 4-360580/1992 (Tokukaihei 4-360580) "FIELD EFFECT TRANSISTOR AND PRODUCING METHOD THEREOF" discloses a method for protecting Al such that a surface of a electrode composed of Al is anodically oxidized and thus the surface is covered with an Al.sub.2 O.sub.3 film.
However, in general, in the driver circuit for driving liquid crystal, it is necessary that a signal outputted from the source/drain electrode wiring 108 of one TFT is inputted to the gate wiring 104 of another TFT and that a certain signal wiring leaps another wiring and is connected to the gate wiring 104 or the source/drain electrode wiring 108 of a certain TFT. For this reason, in the case where the gate wiring 104 and the gate electrode 105 using Al are protected by anodically oxidizing Al, in order to connect the respective electrodes in the above manner, it is necessary that Al.sub.2 O.sub.3 formed on the gate wiring 104 and the gate electrode 105 is etched.
However, since the Al.sub.2 O.sub.3 film has high chemical resistance, in a wet-etching method, selectivity of the Al.sub.2 O.sub.3 film is inevitably smaller than of an Al film, and thus poor contact occurs due to over-etching of Al. Moreover, in dry-etching, since a gas used for etching the Al.sub.2 O.sub.3 film (for example, a mixed gas of BCl.sub.3 and Cl.sub.2) is an etching gas for Al, the selectivity of Al.sub.2 O.sub.3 to Al is reduced and thus poor contact occurs due to the over-etching of Al. Further, in the case of the dry-etching, the etching rate of the Al.sub.2 O.sub.3 film is low, and thus the etching process requires much time.
Therefore, because of difficulty in etching of the Al.sub.2 O.sub.3 film, it is hard to use Al for the gate wiring 104 and the gate electrode 105.